1. Field of the Invention
This invention relates to a sheet supplying/feeding device which is employed in an imaging device such as a copier, printer, facsimile or a composite machine of these devices using electrophotography, and more particularly to a sheet supplying/feeding device which can supply/feed a sheet stably and intermittently with no adverse effect and occurrence of impact sound due to shock during supplying/feeding of the sheet.
2. Description of the Related Art
Traditionally, in an imaging device such as a copier, printer, facsimile or a composite machine of these devices using electrophotography, such a configuration as shown in FIG. 12 has been mainly employed as a sheet hand-supplying/feeding device. This sheet supplying/feeding device 100, as seen from FIG. 12, is provided with a pressing plate 101 on which a sheet (not shown) is placed. This pressing plate 101 is urged upward by a coil spring 102. Above the pressing plate 101, a rotating shaft 103, which is journaled on a bearing of a device body (not shown), is arranged. Crescent-shaped supplying/feeding rolls 104 for supplying/feeding the sheet are fixed to the rotating shaft 103. Core rolls 105 for determining the uppermost stage of the sheet are also rotatably journaled on the rotating shaft 103.
Cam plates 106 are fixed to both ends of the rotating shaft 103, respectively. While the sheet is not supplied/fed, the cam plates 106 depress flanges 107, respectively so that the sheet (not shown) placed on the pressing plate 101 is separated from the supplying/feeding rolls 104.
Meanwhile, the above conventional sheet supplying/feeding 100 has the following problems:                (1) An adverse effect on image quality is generated owing to shock, and the impact sound is also loud.        (2) The intermittent driving of the supplying/feeding rolls 104 which serve as feeding units is likely to be unstable (the rolls do not rotate or continue to rotate).        
The problem of (1) is attributed to the following fact. As seen from FIG. 12, when the cam plates 106 are released, the pressing plate 101 with the sheet placed thereon, which has been depressed by the cam plates 106, is rebounded upward owing to the urging force of the coil spring 102. Then, the sheet hits against the supplying/feeding rolls 100. In the case of continuous supplying/feeding, the shock at this instant when the sheet hits against the supplying/feeding rolls 100 has an adverse effect on the image quality of the sheet in printing which has been supplied/fed just before. The impact sound at this time is also loud.
On the other hand, the problem of (2) is particularly remarkable when a tooth-lack gear 108 is employed, as shown in FIG. 12, as an intermittent driving unit for intermittently driving the supplying/feeding rolls 104 which is the feeding unit. In many cases, in order to meet the demand of cost reduction, recent imaging devices such as a printer employ the tooth-lack gear in place of a spring clutch as the intermittent driving unit for intermittently driving the supplying/feeding rolls 104. The spring clutch, in which gears always in mesh with each other, facilitates coupling/release of driving. On the other hand, where the tooth-lack gear 108 is employed, the gear 108 must be rotated until it meshes with an idler gear 109. The rotating force is generated by the elastic member such as a coil spring 110 attached to the tooth-lack gear 108. Further, the cam plates 106 serving to make the contact/separation of the pressing plate 101 for the supplying/feeding rolls 104 are attached to the rotating shaft 103 of the supplying/feeding rolls 104 which are the feeding units. In a stand-by state, the coil spring 102 which is an urging unit for the pressing plate 101 is in a state where it is compressed to the maximum with strong urging force. Therefore, where the rotating shaft 103 of the supplying/feeding rolls 104 is operated from the stand-by state, very large resistance is generated between the cam plates 106 and the flanges 107 so that the force of the coil spring 110 for the tooth-lack gear 108 must be set at a large magnitude. If this force is too great, the lug (not shown) attached to the tooth-lack gear 108 is likely to be released from a solenoid 111. As a result, the supplying/feeding rolls 104 are not rotated intermittently but rotated continuously. Accordingly, a sheet of paper is not supplied at regular intervals. This leads to paper jamming.
On the other hand, if the force of the coil spring 110 for the tooth-lack 108 is too small, even when the solenoid 111 is operated, the tooth-lack gear 108 starts to operate with a time lag, or otherwise does not rotate. As the case may be, this leads to an inconvenience of causing the shifting of the position where an image is started to be written or paper jamming. Incidentally, a configuration has been also proposed in which pressing force of a separating member acts on the rotating shaft 103 of the supplying/feeding rolls 104. However, the pressing force is smaller than that of the urging member for the pressing plate 101.
Examples of the techniques capable of solving the above problems have been disclosed in JP-A-2000-136035, JP-A-1-308339 or JP-A-8-268574.